1. Field of the invention.
This invention relates to data readout systems, and more particularly, to a data readout system employing a radio frequency data link to permit the transmission of data between an interrogate source and a plurality of data sources located remotely from the interrogate source.
2. Description of the prior art.
Various types of data transmission systems have been proposed for the purpose of obtaining information from a plurality of remote data sources. Certain ones of these systems employ direct communication links established over electrical power lines or telephone lines, for example, connected between transponders associated with the data sources and interrogate apparatus at the interrogate source which controls the readout of the information provided at the data sources.
Other systems have employed wireless communication links to enable the transfer of data from a plurality of data sources to an interrogate source. In the case of wireless data transmission systems, the interrogate source includes a transmitter which generates amplitude or frequency modulated interrogate signals for transmission to the locations of the data sources to be interrogated. Each data source has an associated transponder which includes a receiver for receiving the interrogate signal. The receiver is responsive to the interrogate signals to enable data signals representing the information provided by the data sources to control a transmitter of the transponder to generate reply data signals representing the information. Where amplitude or frequency modulated signals are used, the transmitter includes an oscillator which generates a carrier signal at a given frequency which is modulated by tone signals to provide modulated signals coded to represent the information to be transmitted to the interrogate source. The interrogate source includes a receiver tuned to receive the reply signals.
Where the transmitter is operating in the UHF frequency band or higher a typical oscillator stability could result in transmitter frequency variations on the order of 4 MHz. A conventional receiver having a bandwidth of 25 KHZ would experience some difficulty in locating the transmitted signal. If the receiver bandwidth were increased, a decrease in sensitivity would result and the possibility of interfering signals being in the bandpass with the desired signal would also be increased.
Thus a crystal controlled oscillator is normally required to enable the generation of reply signals at a known frequency to assure proper reception of the signals by the interrogate source receiver.
To enhance the detection capabilities of the interrogate source receiver, it is desirable to provide multiple reply signals. In systems employing transponders having conventional FM transmitters, the carrier signal is usually modulated by a sine wave signal.
This process generates a carrier and several sidebands, the amplitude and number of which is determined by the modulation index. to recover the original modulating frequency requires the demodulating receiver have a bandpass sufficient to receive all predominant sidebands transmitted. Sidebands having an amplitude less than 1% of the unmodulated carrier are usually ignored. Where a higher degree of distortion of the demodulated signal can be tolerated only the predominant sidebands need be considered. It is evident that for the transmitted signal to be properly detected, the receiver must have sufficient bandwidth and be precisely tuned to the transmitter frequency. It is also important that the modulation index of the transmitter (the ratio of the transmitter carrier deviation to the modulation frequency) be sufficiently stable. Should the modulation index increase substantially, the receiver bandpass will be insufficient to receive the predominant sidebands and distortion of the demodulated signal will result. Receiver tuning will also become more critical. Should the modulation index decrease, a higher signal level will be required at the receiver input to provide the same signal-to-noise ratio out of the discriminator.
When the interrogate source, for example, is moving relative to the transponder, peaks and nulls in the received signals occur as a result of the arrival of several rays each traveling over a different path length to the interrogate source receiver from the transponder transmitter. When such rays arrive in phase a peak in the received signal occurs. When such rays are 180.degree. out of phase, a null occurs. A difference in path length between the direct and reflected ray of one-half wavelength is all that is required to cause the received signal to go from a peak to null. This corresponds to a distance difference of 0.375 meters at a transmitter frequency of 400 MHz. Since these nulls and peaks are frequency dependent, a null can occur at one frequency whereas a peak can occur at a frequency 4 MHz away, depending on the path length.
Accordingly, it would be desirable to provide a remote readout system which enables the transmission of data from a data source to an interrogate source wherein data recovery is not dependent upon transmitter and receiver stability and wherein the effects of interference and signal level variation with motion are minimized.